With the advent of relatively inexpensive laser diodes, and of the ability to manufacture optical fibers, both single mode and multimode, having essentially uniform characteristics over virtually indefinite lengths, the use of optical fibers to transmit information, in the form of a modulated light beam launched into the fiber from a laser diode, has become common. For example, fiber optic telephone systems have progressed beyond the experimental stage, and it is also known to use optical fibers for transmission of video signals. The laser diode that is used for launching the light beam into the optical fiber must be in optically-coupled relationship with the proximal end face of the optical fiber. Optimum optical coupling requires that the end face of the fiber be positioned accurately relative to the light-emitting region of the laser diode, but this is difficult because of the small dimensions involved. Typically, the light-emitting region of a multimode laser diode is 0.5-1 .mu.m by 10-15 .mu.m, and the core of a multimode fiber is about 50-100 .mu.m in diameter. These problems are even more serious in the case of a single mode fiber, where the diameter of the core is about one-fifth to one-tenth of that of a multimode fiber and the light-emitting region of the diode is about 0.5 .mu.m by 2 .mu.m, yet single mode fibers are being used increasingly owing to their reduced dispersion and consequent reduced signal degradation. Moreover, it is not only necessary that the fiber be accurately aligned with the light-emitting region of the diode with respect to directions transverse to the longitudinal axis of the fiber (x and y axis positioning) but also that the distance between the proximal end face of the fiber and the diode be accurately controlled (z axis positioning) in order that the maximum amount of light emitted by the diode be coupled into the fiber. It is also desirable that the diode be mounted in a hermetically sealed enclosure, in order to minimize contact with materials that might adversely affect the operation of the diode.
It is known to mount a laser diode inside a conventional TO5 package provided with a transparent window. In this manner, the diode is adequately protected from external influences. In order to maximize the amount of light that is emitted through the transparent window, a sapphire ball is also mounted inside the package, with the light-emitting region of the diode at the focal point of the ball. Thus, a collimated beam of light is generated, the diode and the sapphire ball being so positioned that this collimated beam is directed out of the package through the transparent window. The collimated beam is focused by a second sapphire ball, outside the package, on the end face of the core of an optical fiber. This type of packaging requires alignment of three elements relative to the diode, i.e. the two sapphire balls and the fiber, and consequently fabrication of the package is time-consuming, labor-intensive and expensive.